1. Technical Field
The present invention relates to a switched reluctance motor.
2. Description of the Related Art
In a general switched reluctance motor (SRM), both of a stator and a rotor have magnetic structures, which is salient poles.
In addition, the stator has concentrated type coils wound therearound, and the rotor is configured only of an iron core without any type of excitation device (winding, permanent magnet, or the like), such that the competitive cost is excellent. Further, a speed changeable switched reluctance motor stably generates the continuous torque with the aid of a converter using power semiconductors and a position sensor and is easily controlled to be appropriate for the performance required in each application.
In addition, the switched reluctance motor is inexpensive due to a simple structure of the rotor; however, the switched reluctance motor has disadvantages because a converter formed of semiconductor switches should be used in order to generate the reluctance torque, a cost of the entire system increases, and an expensive control circuit capable of performing calculation at a high speed should be included in order to perform an appropriate control at the time of high speed driving.
Further, a universal motor that is mainly used in the field such as a vacuum cleaner, an electric power tool, or the like, generates the torque without a converter and a position sensor through the use of a commutator and a brush, which is a simple mechanical structure. The universal motor has widely been used in the field such as a vacuum cleaner, an electric power tool, or the like, due to an advantage in which it has a low cost structure rather than improvement in performance by a control. However, coils are wound around a rotor as well as a stator, such that the material cost increases and copper loss of the rotor occurs, thereby reducing efficiency of the motor. Therefore, it is difficult to apply the universal motor to a high level model requiring high efficiency.
FIG. 4 is a schematic configuration view of a switched reluctance motor according to the prior art. As shown, the switched reluctance motor 10 in which only a single phase is shown includes a rotor 11, a stator 12a provided with a stator pole 12b, and a coil 13 wound around the stator pole 12b. 
In addition, when a current is applied to the coil 13, a magnetic field is generated in the stator pole and attractive force is generated between the stator pole 12b and the rotor 11, such that the rotor 11 rotates.
In addition, when a plurality of phase windings are wound around a plurality of stator poles, each of the plurality of phase windings of the stator poles is excited to generate a torque, thereby rotating the rotor.
In this case, a position sensor is required since feedback for a position of the rotor is required, and a converter formed of power semiconductors is required in order to apply a current to the winding of the stator according to the position of the rotor. Furthermore, a controller including a digital signal processor (DSP), a microcontroller unit (MCU), or the like, mounted therein is required in order to perform complex and rapid calculation.
As described above, since the switched reluctance motor according to the related art should necessarily include the converter, the controller, and the position sensor in order to drive the motor, the switched reluctance motor may not be implemented at a low cost, a degree of freedom in design is deteriorated due to a complicated technical structure, and the possibility that a fault or an error will be generated is high.
FIGS. 5 to 8 show a switched reluctance motor according to the prior art. The switched reluctance motor according to the prior art includes a commutator segment and a brush mounted therein in order to remove a control device such as a semiconductor current converting device, an inverter, and the like, for controlling the switched reluctance motor. In the switched reluctance motor according to the prior art, since an advance angle and a dwell angle are mechanically fixed, self-starting may be performed by increasing a contact angle at the time of starting. However, because the advance angle and the dwell angle cannot be changed at the time of high speed driving, a flat-topped current may not be established and efficiency may not be improved.
FIG. 5 is a schematic configuration view of a switched reluctance motor. As shown, the switched reluctance motor 20 includes a rotor 21, commutators 22, brushes 23, coils 24, and a stator 25.
More specifically, the rotor 21 includes the commutators 22 connected to both ends thereof, and is short-circuited to the commutators 22.
Here, the rotor is connected to two commutators 22 so that a central axis thereof coincides with those of the two commutators 22. In addition, the stator 25 and the rotor 21 is a salient pole type.
Further, two pairs of brushes 23 facing each other are provided and are fixed to the stator 25, the stator 25 includes two pairs of stator poles facing each other. In addition, each of two-phase coils 24 is wound around the stator pole.
The rotor 21 rotates to thereby mechanically contact the commutators 22.
Through the above-mentioned configuration, when the rotor 21 rotates, the commutators 22 having the same axis as that of the rotor 21 rotate together with the rotor 21. When the commutators 22 are positioned at a position at which an A phase winding shown as the coil 24 is to be excited, each of the commutators 22 mechanically contacts the brushes 23, such that a current flows therebetween, and when the commutators 22 are positioned at a position at which a B phase winding shown as the coil 24 is to be excited, each of the commutators 22 mechanically contacts the brushes 23, such that a current flows therebetween.
FIG. 6 is an operating state schematic according to excitation of an A phase coil in a switched reluctance motor.
As shown, as each of commutators 22a and 22b contacts brushes 23a, 23b, 23c, and 23d, a voltage is applied, such that a current flows in coils 24a and 24b, which is an A phase. In this case, since the applied voltage is an alternate current (AC) voltage, each of voltages across each of the commutators 22a and 22b may have different polarities. That is, each of voltages across each of the commutators 22a and 22b may be a positive (+) voltage and a negative (−) voltage or be a negative (−) voltage and a positive (+) voltage.
FIG. 7 is a graph showing an inductance according to a position of a rotor in a switched reluctance motor; and FIG. 8 is a graph showing voltage application according to a position of a rotor.
As shown, a current does not immediately arrive at a desired current value at the time of application of a voltage and is not immediately removed at the time of turn-off of the voltage, due to characteristics of an inductance. Therefore, it is important to design an advance angle for building up a current and a dwell angle at which a voltage is turned off before a negative torque is generated, in a minimum inductance period.
In addition, it is possible to implement roles of the position sensor and the converter according to the prior art through this.
More specifically,
            T      ⁡              (                  θ          ,          i                )              =                  1        2            ⁢              i        2            ⁢                        ⅆ                      L            ⁡                          (              θ              )                                                ⅆ          θ                      ,
Where T indicates a torque, θ indicates a position of a rotor, i indicates a phase current, and L indicate an inductance.
As seen in the above Equation, the torque is determined by a generated current and a change rate in inductance.
Therefore, the advance angle indicates an area from after application of a voltage to before an increase in the inductance. The voltage is applied by the advance angle and the inductance then increases, such that a positive torque area is formed.
Since the switched reluctance motor according to the prior art having the above-mentioned characteristics may not change the advance angle and the dwell angle particularly at the time of high speed driving thereof, it is impossible to drive the switched reluctance motor at a high speed or characteristics are deteriorated at the time of the high speed driving of the switched reluctance motor.